Article Geochemistry
August 2011 Vol.56 No.22: 23712378 doi: 10.1007/s11434-011-4589-6
SPECIAL TOPICS:
Seasonal variations in the Sr-Nd isotopic compositions of suspended particulate matter in the lower Changjiang River: Provenance and erosion constraints MAO ChangPing1*, CHEN Jun1, YUAN XuYin2, YANG ZhongFang3 & JI JunFeng1 1
Institute of Surficial Geochemistry, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China; College of Environmental Science and Engineering, Hohai University, Nanjing 210098, China; 3 School of Earth Science and Resources, China University of Geosciences (Beijing), Beijing 100083, China 2
Received January 21, 2011; accepted May 16, 2011
Suspended particulate matter samples were collected monthly for more than 2 years in Nanjing, China to examine seasonal changes in the Sr-Nd isotopic compositions of the lower Changjiang River (CR). The results indicate that the 87Sr/86Sr ratios of the samples ranges from 0.725352 to 0.738128, and the values of εNd(0) ranges from −10.55 to −12.29. The Sr-Nd isotopic compositions show distinct seasonal variations. The samples had lower 87Sr/86Sr ratios and higher εNd(0) values during the flood season than the dry seasons. The seasonal variations primarily reflect the controls of provenance rocks and erosion in different sub-catchments. The relative decrease in 87Sr/86Sr ratios and the increase in εNd(0) values during the flood season may reflect an increasing in the mechanical erosion rate in the upper basin and the contribution of more sediment from the upper reaches. The end member values of 87Sr/86Sr and εNd(0) of the samples were 0.728254 and 11.26, respectively. Changjiang River, suspended particulate matter, Sr-Nd isotopic compositions, seasonality, provenance Citation:
Mao C P, Chen J, Yuan X Y, et al. Seasonal variations in the Sr-Nd isotopic compositions of suspended particulate matter in the lower Changjiang River: Provenance and erosion constraints. Chinese Sci Bull, 2011, 56: 23712378, doi: 10.1007/s11434-011-4589-6
In recent years, major rivers from the Himalayan-Tibetan area have attracted increased interest in the weathering intensity and development history of their drainage basins, as well as changes in their chemical flux to the global ocean because they have recorded the uplift history of the Tibet and Asian monsoon evolution in the Cenozoic period [1–9]. The Changjiang (Yangtze) River (CR) is the third longest (6300 km) river in the world. The CR originates on the Tibetan Plateau at 5100 m and drains into the East China Sea, covering a total area of 181×104 km2, or nearly 20% of the total terrestrial area of China. As the longest river in Asia, the CR runs through several tectonic systems and geomorphic units. The river is also the result of both the Cenozoic Asian topographic regime and the Asian monsoon regime [1,5,10]. *Corresponding author (email:
[email protected]) © The Author(s) 2011. This article is published with open access at Springerlink.com
The CR is characterized by high elevation in the inner part of the catchment basin, a monsoon climate, and intense weathering in most parts of the basin, resulting in large freshwater and sediment discharges. As a result, the CR is one of the most important suspended particulate matter (SPM) transporting rivers in the world. The upper CR basin plays an important role in this process because it supplies most of the suspended sediments discharged to the ocean by the river [11–13]. Under the control of the Asian summer monsoon, water and sediment discharge from both the upper and lower reaches of the CR show seasonal patterns. The precipitation and runoff change seasonally, with 70%–80% occurring from May to October [12]. From 1950–1990 more than 70% of the water was discharged from the CR during summer (May–October), with the average peak occurring in July [14]. The modern Yangtze River discharges most of its annual sediment load between June and Sepcsb.scichina.com
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tember [15]. In addition, a large portion of the CR Basin has a subtropical monsoon climate. Because water discharge is subject to strong seasonality, the suspended particulate matter of the CR varies drastically according to season and from one year to another. Accordingly, several studies have investigated seasonal variations in the geochemistry of dissolved and particulate matter [16–19]. Radiogenic neodymium and strontium isotopes are generally considered to be reliable indicators of the provenance of sediments, not only because geological bodies have different Sr-Nd isotopic compositions that depend on their origins and ages, but also because the Sr-Nd isotopes undergo limited alterations during surficial processes such as weathering and transportation [20–25]. As a result, the Nd-Sr isotopic signatures of the sediments have been useful for identifying their sources and studying erosion processes [2,26–30]. In addition, the Nd-Sr isotopic signatures of geological time scale sediments have been widely used to probe evolution of the Asian monsoon and the uplift history of Tibetan Plateau [31–39]. Several studies of the Nd-Sr isotopic compositions of the CR sediments have recently been conducted [40–42]. However, changes in the seasonal Nd-Sr isotopic compositions of the SPM have not yet been investigated in detail, although a study of modern Amazon Rivers showed that the Sr and Nd isotopic composition of the suspended sediments is seasonally controlled [43]. The present study was conducted to examine the temporal distribution of Sr-Nd isotopic compositions in the lower CR SPM in detail through seasonal sampling and to assess variations in east Asian monsoon rainfall controls based on their seasonal changes. The results of this study will contribute to the overall understanding of the develop-
Figure 1
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mental history of the CR, the continental weathering processes during the Cenozoic period, uplift of the Tibetan Plateau, evolution of the east Asian monsoon, and discrimination of sediment sources in East China and the marginal seas.
1 Materials and methods Seasonal variations were examined by taking monthly SPM samples from the lower stream of the CR at Nanjing (Figure 1) for 2 years. The first year (2005) included only two samples, one from January (dry season), and one from August (flood season). During the second year, samples of SPM were collected monthly from October 28, 2006 through September 24, 2007. All samples were collected at the same location, 32°05′33.9″N, 118°43′27.6″E. Water samples were collected from a boat in the middle of the river channel at a water depth of about 30 cm. Samples were collected in acid-cleaned containers and filtered through 0.45 μm Millipore membranes (herein defined as the size fraction ranging from 63 to 0.45 μm) to collect SPM. The authigenic carbonate minerals may change the Sr isotopic compositions of the detrital sediments of the CR [44]. In this study, all of the samples was selectively dissolved with purified acetic acid solution (0.5 mol/L) at room temperature for up to 24 h in order to remove carbonate minerals. The mineralogy of acid-insoluble samples was then determined using an X-ray diffractometer (XRD), and the dissolution of carbonate minerals in the samples after acetic acid treatment was evaluated based on the XRD analyses (Figure 2).
Geological map of the Changjiang River drainage basin and sampling site (modified from the map in Wing et al. [42]).
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Table 1 Nd and Sr isotopic compositions of the SPM samples at the monitoring stations (Nanjing) Sr/86Sr
2
2005-01-05
0.734557
16
0.512008
2
εNd(0)
12
12.29
2005-08-18
0.727226
6
2006-10-28
0.731997
2
0.512087
8
10.75
0.512026
16
11.94
2006-11-28
0.732020
2
2006-12-18
0.735263
2
0.512040
5
11.67
0.512035
16
2006-12-27
0.734579
11.76
2
0.512046
5
2007-01-19
11.55
0.733670
9
0.512032
4
11.82
2007-02-27
0.729762
2
0.512022
5
12.02
2007-03-22
0.731698
2
0.512019
6
12.07
2007-04-23
0.738128
2
0.512028
3
11.90
2007-05-21
0.733041
2
0.512062
10
11.24
2007-06-22
0.729996
6
0.512044
6
11.59
2007-07-21
0.725352
1
0.512053
8
11.41
2007-08-20
0.727994
2
0.512079
5
10.90
2007-09-24
0.728456
2
0.512097
12
Weighted averagea)
0.728254
87
143
Nd/144Nd
0.512060
10.55 11.26
a) Calculated by averaging monthly Nd and Sr isotopic compositions of the samples according to their monthly fluxes (from 2006 to 2007). Figure 2 X-ray diffraction patterns of selected samples of SPM, bulk samples and acetic acid treated samples.
The pretreated samples were cleaned in pure water, after which they were digested with a mixture of HNO3 + HF solution. Sr and Nd were separated using the standard ion exchange techniques and their isotopic ratios were determined using a Finnigan Triton thermal ionization mass spectrometer at the Department of Earth Sciences, Nanjing University. 87Sr/86Sr was normalized to 86Sr/88Sr=0.1194 and 143Nd/144Nd was normalized to 146Nd/144Nd=0.7219. The analytical blank was
